The present invention relates to a micro-electromechanical relay that combines clamping cantilever beams with movable shuttle structure to provide strong contact force, latching mechanism, and high standoff voltage.
Micro-electro-mechanical system or MEMS refers to micro devices that typically integrate electrical and mechanical elements on a common substrate or substrate stack using microfabrication technology. The electrical elements are typically formed using metal film deposition and patterning techniques, and the mechanical elements are normally fabricated using micromachining techniques which include deposition, lithographic patterning, and etching of various structural and sacrificial materials. Wafer bonding or mating techniques to form multi-layer substrate stack is also commonly used in the fabrication of MEMS devices. Examples of MEMS devices include accelerometers, pressure sensors, micro mirror arrays and MEMS switches to name a few.
MEMS switches generally include two classes of electrical switching devices. One class of the MEMS switches relies on capacitive coupling to switch a radio frequency or microwave signals. This type of MEMS switches only works at high frequencies. The other class of switching devices utilizes metal-metal contact to accomplish the electrical switching function. This class of MEMS switching devices works at DC as well as RF and microwave frequencies, and is usually referred to as micro-electromechanical relays.
Micro-electromechanical relays are inherently small and potentially low cost devices when compared with the conventional electromechanical devices. Micro-electromechanical relays are also capable of high performance over a wide frequency range in terms of insertion loss, isolation, and response linearity, particularly when compared to transistor and diode types of devices. Many of the micro-electromechanical relays developed use electrostatic actuation to deflect cantilever beams or some type of suspended deformable structures for switching actions. The cantilever beams or the suspended deformable structures usually have metal members attached which either serve as part of the conductor terminals or simply a metal bar to short the conductor terminals electrically. The electrostatic actuation method has the advantage of low power consumption and relatively fast switching time but suffers from low contact force inherent to this actuation method. Low contact force corresponds to small contact area and high electrical resistance at the contact, limiting the power level and the lifetime of the micro-electromechanical relay. The physical gap between the cantilever beam and the conductor terminals in the “off” state of the relay is typically on the order of a few micrometers in order to keep the actuation voltage reasonably low. This however makes the relay more susceptible to “self-actuation” caused by voltage spikes in the control lines or high voltage component carried in the signal lines. Examples of MEMS cantilever beam type of relays using electrostatic actuation method are disclosed in U.S. Pat. No. 5,258,591 entitled “Low inductance cantilever switch”, in the name of inventor Buck, and U.S. Pat. No. 5,578,976 entitled “Micro electromechanical RF switch”, in the name of inventor Yao.
The amount of power or current the micro-electromechanical relay can handle is not only limited by the contact resistance of the relay, the overall electrical resistance of the device also has to be kept low in order to minimize the power loss to the relay device itself. Most of the micro-electromechanical relays use thin-film conductors with thicknesses on the order of 1 μm or so for the signal terminals which tend to have relatively high values of electrical resistance for the whole device, regardless of the actuation method. A possible solution to this problem is to increase the conductor thickness to the range of 10-50 μm to reduce the overall resistance of the relay and make it robust. Electroplating is one process technique that can produce such conductors.
Other actuation methods such as shape memory alloy (SMA), electromagnetic, and thermal actuations have also been used in various designs of micro-electromechanical relays. Thermally actuated micro-electromechanical relays can usually provide the high contact force desired and the contact resistance of this type of micro-electromechanical relays can be very low. Thermally actuated relays usually have much higher power consumption compared with relays that use electrostatic actuation. U.S. Pat. No. 4,423,401 issued to Mueller described an early example of a thermally actuated micro-electromechanical relay and U.S. Pat. No. 5,955,817 issued to Dhuler et al is a more recent example of thermally actuated micro-electromechanical relay.
Most of the micro-electromechanical relays require continued application of the actuation voltage or current in order to maintain the relay in at least one of the desired “on” and “off” positions. The only exceptions are those switches that are bi-stable and capable of latching into “on” and “off” positions mechanically. Latching or bi-stable relays have the advantage of reduced power consumption as the only time power is required is during switching. Latching switches are also immune to power failures which is a feature needed by many applications.
An example of thermally actuated bi-stable micro-electromechanical relays is disclosed in U.S. Pat. No. 6,239,685 entitled “Bistable micromechanical switches”, issued May 29, 2001, in the name of inventors Albrecht and Reiley. The relay has a bi-material beam actuator which relies on controlled level of built-in stress and differential thermal expansion coefficients in the bi-material stack to make the relay bi-stable. The bi-material beam in the MEMS relay described in this patent is clamped at both ends and has a limited travel distance between the “on” and “off” state which means the device will have a fairly low standoff voltage. U.S. Pat. No. 6,753,582 issued to Ma disclosed another example of thermally actuated bi-stable micro-electromechanical relays, where a pair of in-plane (lateral) movement thermal actuators is used to push a vertical leaf spring structure (a pre-deformed beam) to provide the snap action of a bi-stable switch.
One challenging issue with the use of double-clamped beam structures having built-in stress is the ability to control and maintain the stress level in the beams during the microfabrication process. The built-in stress is often achieved through deposition of films with a desired stress level, which is often difficult to control from run to run. In addition, subsequent process steps may also alter the stress level within the films, making it even more difficult to maintain the stress at a certain level in these structures. Another issue with this approach is the limited vertical travel distance of the double-clamped beam which corresponds to lower standoff voltage. Use of a pre-deformed vertical leaf spring described in U.S. Pat. No. 6,753,582 eliminates the above-mentioned problems. However, the difficulty with this approach lies in getting good profile and smooth surface finish on the vertical wall of the electrode structures required for good metal-metal contact.
U.S. Pat. No. 6,684,638 issued to Quenzer and Wagner described a micro actuator arrangement for bi-stable micro-electromechanical relay. The micro actuator arrangement combines two or more thermomechanical actuators to achieve high contact force and mechanical latching. The thermomechanical actuators are made of a single material such as electroplated nickel and are disposed on a semiconductor substrate. The micro actuator arrangement is comprised of one lateral actuator that produces movement parallel to the substrate surface in response to thermal stimulation, and one vertical actuator that produce movement perpendicular to the substrate surface in response to thermal stimulation. In the disclosed arrangement, the vertical actuator is a single beam fixed at both ends (also known as double-clamped beam) that can buckle upward in response to a temperature increase.
In general, the designs proposed and developed thus far by various groups do not have the design flexibility for the micro-electromechanical relay to provide low contact resistance, high power handling, and high stand-off voltage in the same device. Furthermore, the fabrication methods proposed so far rely on building the required electrical and mechanical elements on top of a single substrate to realize the device, an approach that is not always flexible enough to address all the design and fabrication issues. Thus, there remains a need for micro-electromechanical relays that are capable of latching, low contact resistance, high power, and high standoff voltage, as well as more flexible ways to fabricate such devices.